KVM: selftests: Test that values written to Hyper-V MSRs are preserved

Enhance 'hyperv_features' selftest by adding a check that KVM
preserves values written to PV MSRs. Two MSRs are, however, 'special':
- HV_X64_MSR_EOI as it is a 'write-only' MSR,
- HV_X64_MSR_RESET as it always reads as '0'.
The later doesn't require any special handling right now because the
test never writes anything besides '0' to the MSR, leave a TODO node
about the fact.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20221013095849.705943-7-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>